Government researchers have found concentrations of pesticides at these sites that have been banned in the U.S. for decades due to health and safety risks. The damage from pesticides also extends beyond the grow sites, as liquid pesticide that is added to irrigation systems can seep into the soil. From there, the pesticides can penetrate nearby waterways, where it can poison aquatic life as well as contaminate public drinking water sources.

Researchers say that it is common to find dead squirrels, birds, and even bears at these sites. “Some of the [pesticide] is so concentrated animals die in a matter of minutes, if not seconds,” Forest Service researcher Craig Thompson said. The pesticides continue to carry on through the food chain as scavengers consume the poisoned carcasses. Officials reported finding a dead fox at one site, along with a dead vulture that had been feeding on it, and dead insects that had been feeding on both.

Not only are the sites full of toxic chemicals, some of the growers leave booby traps at the site, which can pose a serious risk to hikers who encounter the sites by accident. Officials recommend that if you encounter an illegal grow site, you should take note of what you see, then leave the area the way you came in, retracing your steps. Once clear of the site, you should contact the Forest Service or local law enforcement as soon as possible.

Because some of these sites may be occupied by armed growers looking to keep their grow a secret, officials recommend that you avoid lingering at the site or calling attention to yourself. You should also avoid touching anything that looks out of the ordinary, to avoid pesticide contamination.